The present invention relates to encapsulations for phase change material to be used in a thermal management system for a vehicle's electric battery or set of electric batteries. More particularly, the present invention relates to encapsulations which place phase change materials in thermal contact with a vehicle's electric batteries to remove heat generated by the batteries during operation and either discharge it or store it for later use.
Designers of electrical systems for vehicles and the like have recognized that conventional lead acid or nickel cadmium batteries and the like may lose thirty percent or more of their cranking power, operational capacity, and charge acceptance as temperatures in the immediate operating environment of the battery reach levels beyond the preferred thermal envelope. Furthermore, fluctuating battery temperatures can cause uneven thermal gradients across the battery, compounding the operational shortfalls. The life expectancy of a lead-acid battery, for example, decreases if the temperature in the immediate operating environment rises above 27.degree.-30.degree. C. for an extended period, and decreases substantially if temperatures rise above 45.degree. C.
With the advent of hybrid and electric vehicles, this problem has taken on even more significant proportions. Most current designs for electric cars call for banks of electric batteries which must be packed into single or multiple closed compartments. These batteries generate substantial amounts of heat during operation, particularly where high power is required, such as during hill climbing, acceleration, and regenerative braking. This heat, if it cannot be dissipated quickly and uniformly, adversely affects the performance of the batteries. Thus, there is a need to provide a system for thermal management of electric batteries in order to offset this detrimental temperature rise.
There are primarily two existing methods to thermally manage batteries: forced convection of air over the surface of the battery, and the circulation of heat transfer liquids around the battery. On a hot summer day, the air flow may be too warm to absorb any heat from the battery. Conversely, on a cold winter day, the battery may in fact be overcooled to a temperature far less than desired. For air circulation to be effective, air passages must be provided between batteries to allow for air flow while the vehicle is in motion, or a supplemental fan must be used. Both of these methods require additional ducting, control systems, and other complexities. Additionally, the heat transfer from air to a battery wall is often not adequate for quick cooling or heating.
Thermal management systems which circulate heat transfer liquid around a battery also have disadvantages. These systems typically require circulating water (if not an antifreeze solution), pumps, a radiator, and a variety of controls. These components significantly raise the cost, size, weight and complexity of the vehicle.
Another key problem with existing thermal management systems is their inability to maintain optimum, uniform temperatures within a battery block. It is very important that batteries be kept within specified temperature ranges if optimal battery performance and long life are to be achieved. When multiple batteries are used in a system such as those in an electric vehicle this problem becomes more apparent. Temperature differences between the cells of the batteries lead to capacity differences in these cells. This imbalance causes some cells to be either over discharged or overcharged during vehicle operation which then leads to premature failure of the battery.
Some previous thermal management systems have not even attempted to maintain uniform temperatures over extended time periods, but rather have focused on providing instantaneous warming. Some such battery warmers have used phase change materials ("PCMs") to instantaneously raise the temperature of the battery electrolyte to a desired level. PCMs, alone or combined in eutectic mixtures, release or store heat of transition as they change phase between liquid and solid. PCMs typically have high latent heats of fusion such that significant amounts of energy can be "stored" as the solid PCMs melt, while significant amounts of energy can be dissipated to a heat sink or the like as the liquid PCMs solidify.
For example, U.K. Patent Application 2,125,156 discloses the use of PCMs in sealed bags in the battery electrolyte or even contained in the separator plates to place the PCM in intimate thermal contact with the electrolyte. When the liquid PCM discharges heat to the surrounding electrolyte over a short period of time, the temperature of the electrolyte rises to a desired level. The PCM gradually undergoes phase change to the solid state and must be reheated to be used again. Another is example is Japanese Patent Application No. S43-22512.
According to the present invention, an encapsulation apparatus is provided for a thermal management system for at least one electric battery. The encapsulation apparatus comprises a housing including an inner wall defining a receptacle for the at least one battery. The inner wall being positioned in thermal contact with the at least one battery. The apparatus further includes an outer wall positioned in spaced-apart relationship with the inner wall to define an enclosure therebetween for receiving a phase change material. Advantageously, the encapsulation operates in the thermal management system to absorb heat generated by the battery during vehicle operation, frequent cycling, and rapid charge, so that the temperature of the battery electrolyte remains within an acceptable range, avoiding performance losses associated with high temperature battery operation. In addition, the encapsulated PCM operates to prevent the temperature of the battery electrolyte from falling below a critical range during periods of inactivity or in cold weather environments contingent upon the amount of heat in storage and ambient conditions.
Further in accordance with the present invention, an encapsulation apparatus is provided for a thermal management system for at least one electric battery, the electric battery including a case having a plurality of faces. The encapsulation apparatus comprises at least one panel formed to include an interior region for receiving a phase change material, and means for affixing the at least one panel to a face of the electric battery case to place the phase change material in heat transfer relationship with the battery case.
Yet further in accordance with the present invention, a battery case is provided. The battery case comprises an exterior shell, and an interior shell positioned in a spaced apart relationship with the exterior shell to define a region for receiving phase change material.
Additional objects, features, and advantages of the invention will become apparent to those skilled in the art upon consideration of the following detailed description of preferred embodiments exemplifying the best mode of carrying out the invention as presently perceived.